Optical fibers used in optical communications have wavelength dispersion characteristics in which the transmission speed varies according to the wavelength of light which is transmitted. Therefore, when lasers oscillating at a plurality of wavelengths are used as light sources, the signal waveforms unfavorably vary during the transmission.
As lasers used in the optical transmissions, DFB-LDs (Distributed Feedback Laser Diodes) having a diffraction grating in the vicinity of an active layer are known. FIG. 8 is a perspective view illustrating an example of the conventional DFB-LDs. In FIG. 8, reference numeral 1 denotes an n type semiconductor substrate. A diffraction grating 8 is formed on the n type semiconductor substrate 1. An n type waveguide layer 9, an active layer 10, a p type cladding layer 11, and a cap layer 12 are successively grown on the diffraction grating 8. A DFB laser thus produced achieves single mode oscillation.
When the diffraction grating is uniform in the DFB laser, however, the laser fundamentally oscillates in two longitudinal modes, and the intensity ratio of the two modes varies according to the phase of the diffraction grating at the laser facet. Therefore, whether a laser oscillating at a single wavelength is achieved or not becomes a matter of probability.
On the other hand, a DFB laser with a .lambda./4-shifted diffraction grating, in which the phase of the diffraction grating is shifted by .lambda./4 in the center of the laser resonator, stably oscillates at a single wavelength regardless of the phase of the diffraction grating at the laser facet.
Such a .lambda./4-shifted diffraction grating is usually formed by a two-luminous-flux interference exposure method.
The two-luminous-flux interference exposure method will be described with reference to FIG. 9.
In FIG. 9, laser light 14 is emitted from a light source, for example, an Ar laser 13. The laser light 14 is reflected by a mirror 15, expanded by a beam expander 16, and divided into two light fluxes by a beam splitter 17. The two light fluxes are reflected by movable mirrors 18 and strike a substrate 1 at an angle .theta.. At this time, interference fringes are generated on the substrate 1 due to the incident light, and a period cycle .LAMBDA. of the interference fringe is defined by .LAMBDA.=.lambda./2sin.theta., where .lambda. is a wavelength of the light source.
FIGS. 10(a)-10(k) are cross-sectional views schematically illustrating process steps in a method of producing the conventional .lambda./4-shifted diffraction grating. In the figures, a positive type photoresist 2, which is patterned into a form of a diffraction grating, is disposed on the substrate 1. The thickness of the photoresist 2 is below 100 nm and this is thinner than the height of a node relative to the substrate (about 120 nm) in the thickness direction, in the light intensity distribution generated in the photoresist used in the conventional production of the diffraction grating. Reference numeral 4 designates an SiNx film and reference numeral 3 designates a photoresist for protection. Reference numeral 5a designates a region where a diffraction grating, whose phase is opposite to the diffraction grating formed using the photoresist 2 as a mask, is produced. Reference numeral 5b designates a region where a diffraction grating, whose phase is the same as that of the diffraction grating formed using the photoresist 2 as a mask, is produced. Reference numeral 6 designates a phase shift region.
A description is given of the method of producing the .lambda./4-shifted diffraction grating.
Initially, a positive type photoresist is applied to the semiconductor substrate to a thickness below 100 nm. Then, the photoresist is exposed by the two-luminous-flux interference exposure, followed by development, resulting in the photoresist 2 patterned into the form of a diffraction grating (FIG. 10(a)). The pattern of diffraction grating has a period of about 0.2 micron.
Then, an SiNx film 4 about 30 nm thick is formed on the substrate 1 by chemical vapor deposition utilizing electron cyclotron resonance (hereinafter referred to as ECR-CVD) (FIG. 10(b)).
Then, a protective resist 3 is selectively formed on the SiNx film 4 in an opposite-phase region 5a, i.e., a region where a diffraction grating whose phase is opposite to the diffraction grating pattern of the photoresist 2 is produced (FIG. 10(c)). Then, using the protective resist 3 as a mask, the SiNx film 4 in a same-phase region 5b, i.e., a region where a diffraction grating whose phase is the same as the diffraction grating pattern of the photoresist 2 is produced, is selectively removed (figure 10(d)). Then, the substrate 1 is etched using the photoresist 2 patterned into the form of diffraction grating as a mask (FIG. 10(e)).
After removing the protective resist 3 (FIG. 10(f)), the SiNx film 4 on the photoresist 2 is selectively etched away with hydrofluoric acid, utilizing a characteristic that the etching rate of the SiNx film 4 on the photoresist 2 is higher than the etching rate of the SiNx film 4 on the substrate 1 (FIG. 10(g)). After the etching, the photoresist 2 is removed (FIG. 10(h)).
Then, a protective resist 3 is selectively formed on the substrate 1 in the same-phase region 5b (FIG. 10(i)). Then, the substrate 1 in the opposite-phase region 5a is etched using the SiNx film 4 which is formed in the step of FIG. 10(h) as a mask (FIG. 10(j)). Finally, the protective resist 3 and the SiNx film 4 are removed (figure 10(k)). In this way, a region 6 where a phase of the diffraction grating is shifted by .pi. is produced.
In the conventional method of producing the .lambda./4-shifted diffraction grating, when the SiNx film 4 is formed on the photoresist 2 in the step of FIG. 10(b), the photoresist 2 is completely covered by the SiNx film 4, so that it is impossible to remove the photoresist 2 together with the SiNx film 4 disposed thereon by a lift-off technique. Therefore, as shown in FIGS. 10(f) and 10(g), the SiNx film 4 on the photoresist 2 is selectively removed by etching utilizing a difference in etching rates between the SiNx on the photoresist 2 and the SiNx on the substrate 1, whereby the reversal of the phase is achieved.
The conventional method of producing the .lambda./4-shifted diffraction grating requires a lot of process steps, i.e., several resist applying steps, photolithographic steps, etching steps, film forming steps, and the like as illustrated in FIGS. 10(a)-10(k), which cause the disadvantage that the production process is complicated. In addition, since the difference in the etching rates between the SiNx on the photoresist 2 and the SiNx on the substrate 1 is utilized when the SiNx on the photoresist 2 is selectively removed, the production process is unstable.
Meanwhile, Japanese Published Patent Application No. 124767 discloses another method of producing a .lambda./4-shifted diffraction grating. In this method, a positive type photoresist capable of image reversal is applied to a substrate and a light shielding plate is formed on a prescribed portion of the photoresist. Then, the photoresist with the light shielding plate is subjected to two-luminous-flux interference exposure and reversal baking. Then, the light shielding plate is removed and the photoresist is subjected to two-luminous-flux interference exposure again, resulting in a diffraction grating in which the phase is shifted by .lambda./4 with the light shielding plate as a boundary. In this method, however, the step of reversal baking complicates the production process.
Japanese Published Patent Application No. 61-292923 discloses still another method of producing a .lambda./4-shifted diffraction grating. In this method, a photoresist obtained by mixing a positive type photoresist with a negative type photoresist is applied to a substrate, and a material that suppresses radiation sensitivity is implanted into a prescribed region of the photoresist. Then, the photoresist is subjected to two-luminous-flux interference exposure, resulting in a diffraction grating in which the phase is shifted by .lambda./4 with the implanted region as a boundary. In this method, however, the step of implanting the material that suppresses radiation sensitivity complicates the production process.